1. Field of Invention
The present invention relates to a method for manufacturing semiconductors. More particularly, the present invention relates to the method for manufacturing gate oxide layers which have a high breakdown voltage, a low tunneling effect and a low oxide trapped charges.
2. Description of Related Art
In the manufacture of semiconductors, the quality of a gate oxide layer has significant effects on the electrical properties of a metal oxide semiconductor transistor. Excessive amounts of defects, impurities or charges in the gate oxide layer can bring down the breakdown voltage and shorten the time-dependent dielectric breakdown (TDDB) time. As a result, the reliability and lifetime of a metal oxide semiconductor transistor is shortened. Therefore, caution must be exercised during the manufacture of the gate oxide layer so that proper electrical characteristics are kept. Several sources contribute to the charges that affect the electrical properties of a gate oxide layer, they are: (1) differences in interfacial states; (2) fixed oxide charges in an oxide layer; (3) mobile ionic charges; and (4) oxide trapped charges. The oxide trapped charges are formed in places where electrons or holes are trapped by impurities or unsaturated bonds within the gate oxide layer during manufacturing. Therefore, the oxide trapped charges are distributed randomly. At present, methods to increase the breakdown voltage, minimise the tunneling effect and decrease the oxide trapped charges of a gate oxide layer are still major research topics.
FIGS. 1A and 1B are cross-sectional views showing the progression of manufacturing steps in the production of a gate oxide layer according to a conventional method.
As shown in FIG. 1A, a silicon substrate 10 is first provided. The substrate 10 is placed inside an oxidation furnace, and then oxygen is passed. Next, a preoxidation process is performed by heating the furnace to a temperature of about 750.degree.-850.degree. C. to form a first oxide layer 11 on the substrate 10. Thereafter, the substrate 10 is placed inside an oxidation furnace, and then either nitrous oxide or nitric oxide is passed. In a subsequent step, heating is carried out at a temperature of about 750.degree.-850.degree. C. to form a barrier layer 12 at the interface between the first oxide layer 11 and the substrate 10. The barrier layer 12 can be an oxynitride layer.
Next, in FIG. 1B, the substrate 10 is once again placed inside an oxidation furnace and oxygen is passed. Then, a reoxidation process is carried out at a temperature of about 750.degree.-850.degree. C. to form a second oxide layer 13 at the interface between the barrier layer 12 and the substrate 10.
Finally, subsequent conventional processes for the complete formation of a gate oxide layer are performed. Because the conventional processes are familiar to those skilled in the art, detailed descriptions are omitted here.
The above conventional method of forming a barrier layer 12 between the first oxide layer 11 and the second oxide layer 13 is not too effective in increasing the breakdown voltage, minimising the tunneling effect and decreasing the amount of trapped oxide charges. In light of the foregoing, there is a need in the art for improving method of forming a gate oxide layer.